Chapter 7: Heredity

1. Chapter 7: Heredity Overview of this chapter: Mendelian laws Manipulation of Mendelian laws Backcross/Testcross Monohybrid/Dihybrid Cross Beyond Mendelian Inheritance Incomplete Dominance/Codominance Multiple Alleles Gene Interactions Pleiotropy/Epistasis Linked Genes, Cross-over, Linkage Mapping Barr Body Mutations Gene/Chromosomal Mutations, Nondisjunction

2. Gregor Mendel The father of modern genetics is Gregor Mendel - Austrian monk, in 1850s - Bred garden peas in order to study patterns of inheritance - Theory of genetics is one of particulate inheritance - Mendelian Laws: Law of Dominance Law of Segregation Law of Independent Assortment

3. Law of Dominance -When two organisms, each homozygous (pure) for two opposing traits are crossed, the offspring will be hybird (carry two different alleles; Gg) but will exhibit only the dominant trait (G) - Mendel’s first law: Law of dominance - The trait that remains hidden is known as the recessive trait (g)

4. Thank you Jackie for the amazing paint skills. Law of Segregation - During formation of gametes, the two traits carried by each parent separate - Best exemplified by monohybrid cross - Trait that’s not evident in either parent appears in F1 generation

5. Backcross or Testcross - Way to determine genotype of an individual plant or animal showing only dominant trait - Ex.) individual (B/_) is crossed with homozygous recessive (b/b) - If individual is in fact homozygous dominant, all offspring will be B/b and show dominant trait. - No offspring showing recessive trait. - If individual is hybrid (B/b) - one half of offspring show recessive trait - Therefore if any offspring show recessive trait, parent must be hybrid.

6. Law of Independent Assortment - Applies when cross is carried out between two individuals hybrid for two or more traits are not on the same chromosome. - called dihybrid cross. - Law states that during gamete formation, alleles for one trait (e.g. height), segregate independently from the alleles of a gene for another trait (e.g. seed colour) - Only factor that determines how these alleles segregate (assort) is how the homologous pairs line up in metaphase of meiosis I, which is random. - However, if the genes are linked then they will not assort independently.

7. The Dihybrid Cross - Cross between two F1 plants is called a dihybrid cross - Cross between individuals that are hybrid for two different individuals that are hybrid for two different traits (e.g. height and seed colour) - This cross can produce four different types of gametes: TY, Ty, tY, ty (dihybrid cross) - Many different genotypes possible in the resulting F2 generation. - Phenotype ratio of the dihybrid cross, 9:3:3:1, 9 tall, yellow; 3 tall, green; 3 short, yellow; and 1 short, green. F2: 9/16 tall, yellow; 3/16 tall, green; 3/16 short, yellow; 1/16 short, green.

8. Multiple Alleles Codominance - both traits show Incomplete Dominance -Incomplete dominance is characterized by blending. - Neither trait is dominant, thus the convention for writing the genes uses a different convention: all capital letters. -When there are more than two allelic forms of a gene

9. Gene Interactions - Pleiotropy: Ability of single gene to affect organism in several ways. - e.g. autosomal recessive disease; cystic fibrosis. - Epistasis - two separate genes control one trait - but one masks expression of other gene - Gene that masks the expression of the other gene is epistatic to the gene that's masked

10. Polygenic Inheritance Polygenic: genus that vary along a continuum, e.g. skin colour, height, hair colour. Genes and the Environment - Environment can alter expression of genes - Development of intelligence is result of interaction of genetic predisposition and environment - Genus from previous generations “learned” of a way to “better” do something

11. Linked Genes - genes on same chromosomes - Tend to be inherited together and do not assort independently (unless separated by cross-over) Sex-Linkage -Of 46 human chromosomes - 44 are autosome and 2 are sex chromosomes -traits carried on X chromosome are sex-linked -females (XX) inherit two copies of sex-linked genes -males (XY) inherit only one copy of sex-linked gene -mutated X-linked gene is: X- -can lead to sex-lniked traits such as colour blindness, hemophilia and Duchenne muscular dystrophy

12. Important facts about sex-linked traits - All daughters of affected fathers are carriers! -Remember: Sex-linked traits are located on X chromosomes. - Son has 50% percent chance of inheriting sex-linked trait from carrier mother - No “carrier" state for X-linked traits in males. - If male has gene, he will express it! - Uncommon for female to have recessive sex-linked condition (for it to happen, she must inherit mutant gene from both parents)

13. Cross-over and linkage mapping - The farther two genes on chromosomes - more likely to be separated by cross-over during meiosis. - When cross-over/recombination occur, one can see chiasmata (physical bridge built around point of exchange) - Result of cross-over = recombination - Major source of variation! - Map unit: distance within which recombination occurs 1 percent of the time. - Tells of order of linked gene on chromosome (pretty useful for mapping genome) Ex) 1) Genes A, B, D are linked 2) Cross-over or recombination frequencies for B and D is 5%, B and A is 30%, D and A is 25% 3) Draw linkage map from this (name BDA or ADB)

14. Important Tools to Know Recombination Frequency: Number of recombinants ------------------------------------- x 100 Total number of offspring Pedigree:

15. X-Inactivation - Barr body - In development of embryo in female mammal, one of X chromosome is inactivated in every somatic cell - Inactivation occurs randomly - Results in genetic mosaic - Some cells have one X inactivated, some cells have other X inactivated, therefore all cells of female mammals not identical. - Inactivated chromosome condenses into dark spot of chromatin - Barr body Remember: All female body cells have one Barr Body. Normal male cells have none.

16. Mutations Mutations: any changes in the genome - Two types: gene mutations/chromosomal mutations - Chromosomal mutations can be seen with karyotype -Chromosomal aberrations: Deletion, Inversion, Translocation, Polyploidy, (Duplication) Some examples....

17. Nondisjunction Nondisjunction: an error that sometimes occurs during meiosis in which homologous chromosome fail to separate as they should Aneuploidy: Abnormal number of chromosomes Triploid: Extra set of chromosomes (3n) Tetraploid: Organism with (4n) Polyploid: Organism with extra sets of chromosomes - Common in plants - Results in abnormally large size plants - Some cases: responsible for new species E.g Triploid: Extra set of chromosomes (3n) Strawberries (8n)

18. Genomic Imprinting + Extranuclear Genes - Two inheritance patterns that are exceptions to Mendelian inheritance - genomic imprinting + extranuclear genes Genomic imprinting: a variation in phenotype depending on whether a trait is inherited from mother or father. - Occurs during gamete formation - caused by silencing of particular allele by methylation of DNA, therefore zygote expresses only one allele of imprinting gene - Imprint carried to all body cells and passed through generations - Imprinted gene located on autosomes Extranuclear gene are located on mitochondria and chloroplasts - DNA is small, circular, carry small number of genes - Linked to rare/severe inherited genes in humans - Mutations in these genes cause weakness/deterioration in muscles - Mitochondrial DNA is inherited only from mother because father’s mitochondria does not enter egg during formation.

19. THANKS FOR WATCHING!!! Wrapping it up with a cool picture!